Write rclone uploads under a temporary name, re-upload short blobs (closes #266)

The rclone backend wrote each object straight to its key, so killing an
upload to a local or sftp remote left a truncated object there. The next
backup found the key, skipped the upload and recorded a snapshot that
could not be restored.

On a remote where rclone says a file can be seen while it is still being
written, an object is now written under a name ending in `.partial` and
moved onto its key with the remote's server-side move, as rclone's own
copy does; listings skip such names. A backup also uploads a blob again
when the stored object's size differs from the blob's.

Judgement call: the temporary name is used only where rclone sets its
PartialUploads feature; other remotes already show an object only once
complete.

Model: opus-5-5

README.md

@@ -428,6 +428,17 @@ a local or mounted filesystem. Useful for testing or backing up to a NAS.
 **Rclone** (`rclone://remote/path`): Uses rclone's 70+ supported cloud
 providers. Requires rclone to be configured separately (`rclone config`).
 
+An upload cut off part-way leaves nothing under the object's name on S3, which
+shows an object only once its upload has completed, and on the local filesystem
+backend, which writes a temporary file and renames it into place. Rclone
+remotes that show a file while it is still being written (local, sftp, ftp and
+smb among them) are written under a temporary name ending in `.partial` and
+moved into place where the remote has a server-side move; without one they are
+written in place. Other rclone remotes show an object only once its upload has
+completed. A leftover `.partial` file is ignored and can be deleted. A backup
+that finds a blob stored at a size other than the one it packed uploads the
+blob again.
+
 Legacy S3 configuration via `s3.*` fields (endpoint, bucket, prefix, etc.) is
 still supported for backward compatibility. `storage_url` takes precedence if
 both are set.

TODO.md

@@ -22,6 +22,18 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Stopped a backup from trusting a blob object left short by
+  a killed upload
+  ([issue #266](https://git.eeqj.de/sneak/vaultik/issues/266)). The
+  rclone backend wrote each object straight to its key, so killing an
+  upload to a local or sftp remote left a truncated object there; the
+  next backup found the key with `Stat`, skipped the upload and recorded
+  a snapshot that could not be restored. On a remote where rclone says a
+  file can be seen while it is being written, an object is now written
+  under a temporary name ending in `.partial` and moved into place, and
+  listings skip such names. A backup also uploads a blob again when the
+  stored object's size differs from the blob's.
+
 - 2026-10-07: Corrected documentation, help text and comments that were
   false about the code
   ([issue #233](https://git.eeqj.de/sneak/vaultik/issues/233)). A blob

internal/snapshot/scanner.go

@@ -1535,8 +1535,8 @@ func (s *Scanner) handleBlobReady(
 	return nil
 }
 
-// uploadBlobIfNeeded uploads the blob to storage if it doesn't already
-// exist, returns whether it existed
+// uploadBlobIfNeeded uploads the blob to storage unless an object of the
+// blob's size is already there, and returns whether it was there
 func (s *Scanner) uploadBlobIfNeeded(
 	ctx context.Context,
 	blobPath string,
@@ -1546,11 +1546,13 @@ func (s *Scanner) uploadBlobIfNeeded(
 ) (bool, error) {
 	finishedBlob := blobWithReader.FinishedBlob
 
-	// Check if blob already exists (deduplication after restart)
+	// Check if blob already exists (deduplication after restart). An
+	// object of another size was left by an upload cut off part-way and
+	// is uploaded again.
 	destination := s.storage.Info().Location
 
-	_, err := s.storage.Stat(ctx, blobPath)
-	if err == nil {
+	stored, err := s.storage.Stat(ctx, blobPath)
+	if err == nil && stored.Size == finishedBlob.Compressed {
 		log.Info("Blob already exists in storage, skipping upload",
 			"hash", finishedBlob.Hash,
 			"size", humanize.Bytes(safeUint64(finishedBlob.Compressed)))
@@ -1561,6 +1563,11 @@ func (s *Scanner) uploadBlobIfNeeded(
 		return true, nil
 	}
 
+	if err == nil {
+		log.Warn("Blob in storage has the wrong size, uploading it again",
+			"hash", finishedBlob.Hash, "stored_size", stored.Size)
+	}
+
 	s.ui.Beginf("Uploading blob %s (%s) to %s.",
 		s.ui.Hex(finishedBlob.Hash), s.ui.Size(finishedBlob.Compressed),
 		s.ui.Path(destination))

internal/storage/conformance_test.go

@@ -14,8 +14,9 @@ import (
 
 // runStorerConformance is the shared Storer contract. Every backend that
 // can run in-process is expected to pass it: TestFileStorer runs it against
-// file://, TestS3Storer against s3://. A new backend inherits this coverage
-// by passing its own constructor, so the contract is defined once.
+// file://, TestS3Storer against s3://, TestRcloneStorer against rclone's
+// local backend. A new backend inherits this coverage by passing its own
+// constructor, so the contract is defined once.
 //
 // It exercises the public Storer interface: round-trip, stat, list with
 // prefix filtering, overwrite, delete, delete-of-missing, and not-found on

internal/storage/file.go

@@ -50,9 +50,9 @@ const storageDirPerm = 0o755
 // temp file carrying this suffix and only renames it onto the real key once
 // the whole object is on disk, so an interrupted write can never leave a
 // truncated object at the key a later run would Stat and trust as a complete
-// blob. List and ListStream skip these files, so a leftover from an
-// interrupted write is never listed or trusted as a blob; it is otherwise
-// harmless and is overwritten when the same key is written again.
+// blob. The rclone backend's upload does the same on remotes that need it.
+// List and ListStream skip these files, so a leftover from an interrupted
+// write is never listed or trusted as a blob; it is otherwise harmless.
 const tempSuffix = ".partial"
 
 // Put stores data at the specified key.

internal/storage/file_atomic_test.go

@@ -14,6 +14,9 @@ import (
 // errStreamInterrupted stands in for an upload cut off mid-stream.
 var errStreamInterrupted = errors.New("connection reset mid-upload")
 
+// testBlobKey is a key laid out as a blob's key is.
+const testBlobKey = "blobs/aa/bb/aabbccddeeff"
+
 // failingReader yields its data once, then fails.
 type failingReader struct {
 	data []byte
@@ -43,7 +46,7 @@ func TestFileStorer_InterruptedWriteLeavesNoTrustedObject(t *testing.T) {
 	}
 
 	ctx := context.Background()
-	key := "blobs/aa/bb/aabbccddeeff"
+	key := testBlobKey
 
 	err = f.PutWithProgress(ctx, key, &failingReader{data: []byte("partial")}, 4096, nil)
 	if err == nil {
@@ -79,7 +82,7 @@ func TestFileStorer_ListSkipsPartialFiles(t *testing.T) {
 	}
 
 	ctx := context.Background()
-	realKey := "blobs/aa/bb/aabbccddeeff"
+	realKey := testBlobKey
 
 	err = f.Put(ctx, realKey, strings.NewReader("blob-bytes"))
 	if err != nil {

internal/storage/rclone.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"bytes"
 	"context"
+	"crypto/rand"
 	"errors"
 	"fmt"
 	"io"
@@ -68,14 +69,7 @@ func (r *RcloneStorer) Put(ctx context.Context, key string, data io.Reader) erro
 		return fmt.Errorf("reading data: %w", err)
 	}
 
-	// Upload the object
-	_, err = operations.Rcat(ctx, r.fsys, key,
-		io.NopCloser(bytes.NewReader(buf)), time.Now(), nil)
-	if err != nil {
-		return fmt.Errorf("uploading object: %w", err)
-	}
-
-	return nil
+	return r.upload(ctx, key, bytes.NewReader(buf))
 }
 
 // PutWithProgress stores data with progress reporting.
@@ -89,13 +83,7 @@ func (r *RcloneStorer) PutWithProgress(
 		callback: progress,
 	}
 
-	// Upload the object
-	_, err := operations.Rcat(ctx, r.fsys, key, io.NopCloser(pr), time.Now(), nil)
-	if err != nil {
-		return fmt.Errorf("uploading object: %w", err)
-	}
-
-	return nil
+	return r.upload(ctx, key, pr)
 }
 
 // Get retrieves data from the specified key.
@@ -173,6 +161,10 @@ func (r *RcloneStorer) List(ctx context.Context, prefix string) ([]string, error
 
 	err := operations.ListFn(ctx, r.fsys, func(obj fs.Object) {
 		key := obj.Remote()
+		if strings.HasSuffix(key, tempSuffix) {
+			return
+		}
+
 		if prefix == "" || strings.HasPrefix(key, prefix) {
 			keys = append(keys, key)
 		}
@@ -202,6 +194,10 @@ func (r *RcloneStorer) ListStream(
 			}
 
 			key := obj.Remote()
+			if strings.HasSuffix(key, tempSuffix) {
+				return
+			}
+
 			if prefix == "" || strings.HasPrefix(key, prefix) {
 				ch <- ObjectInfo{
 					Key:  key,
@@ -230,6 +226,45 @@ func (r *RcloneStorer) Info() Info {
 	}
 }
 
+// upload writes data to key. On a remote where rclone says a file can be
+// seen while it is still being written (its PartialUploads feature: local,
+// sftp, ftp, smb and others), it writes under a temporary name ending in
+// tempSuffix and moves the object onto key once it is complete, as
+// rclone's own copy does, so a killed upload cannot leave a truncated
+// object at key. List and ListStream skip a temporary object left behind.
+func (r *RcloneStorer) upload(ctx context.Context, key string, data io.Reader) error {
+	features := r.fsys.Features()
+	if !features.PartialUploads || features.Move == nil {
+		_, err := operations.Rcat(ctx, r.fsys, key, io.NopCloser(data), time.Now(), nil)
+		if err != nil {
+			return fmt.Errorf("uploading object: %w", err)
+		}
+
+		return nil
+	}
+
+	tempKey := key + "-" + rand.Text() + tempSuffix
+
+	obj, err := operations.Rcat(ctx, r.fsys, tempKey, io.NopCloser(data), time.Now(), nil)
+	if err != nil {
+		// Rcat returns the object it wrote when the written data fails its check.
+		if obj != nil {
+			_ = obj.Remove(ctx)
+		}
+
+		return fmt.Errorf("uploading object: %w", err)
+	}
+
+	_, err = features.Move(ctx, obj, key)
+	if err != nil {
+		_ = obj.Remove(ctx)
+
+		return fmt.Errorf("moving object into place: %w", err)
+	}
+
+	return nil
+}
+
 // progressReader wraps an io.Reader to track read progress.
 type progressReader struct {
 	reader   io.Reader

internal/storage/rclone_test.go

@@ -1,28 +1,45 @@
 package storage_test
 
 import (
+	"bytes"
 	"context"
 	"errors"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
+	"github.com/rclone/rclone/fs"
 	"sneak.berlin/go/vaultik/internal/storage"
 )
 
-// The rclone backend is a thin adapter over the rclone library: it turns a
-// (remote, path) pair into rclone's "remote:path" string, hands it to
-// rclone, and maps rclone's own results back to the Storer interface. What
-// can be tested in-process, without a configured remote or network, is that
-// adapter layer — how the arguments are shaped and how construction errors
-// are reported. The data-plane operations (Put/Get/List/Delete) are rclone's
-// own, exercised against a real provider (drive, s3-via-rclone, ...), which
-// needs a configured remote with credentials and network access and so is
-// out of reach of a unit test. The shared Storer conformance suite therefore
-// runs against the in-process file and s3 backends; the rclone backend
-// inherits that contract once a remote is configured.
-//
 // These tests use rclone's ":local:" on-the-fly backend, which addresses the
-// local filesystem directly without any configured remote, so construction
-// runs entirely in-process.
+// local filesystem directly without any configured remote, so they run
+// entirely in-process. A remote that needs credentials and network access
+// (drive, s3 via rclone, ...) is out of reach of a unit test.
+
+// newRcloneStorer builds an rclone backend on rclone's local backend,
+// rooted at a fresh temp directory.
+//
+//nolint:ireturn // conformance runs against the Storer interface by design
+func newRcloneStorer(t *testing.T) storage.Storer {
+	t.Helper()
+
+	s, err := storage.NewRcloneStorer(context.Background(), ":local", t.TempDir())
+	if err != nil {
+		t.Fatalf("NewRcloneStorer: %v", err)
+	}
+
+	return s
+}
+
+// TestRcloneStorer runs the shared Storer contract against the rclone
+// backend.
+//
+//nolint:paralleltest // NewRcloneStorer installs the process-global rclone config
+func TestRcloneStorer(t *testing.T) {
+	runStorerConformance(t, newRcloneStorer)
+}
 
 // TestNewRcloneStorerConstruction checks that a valid remote constructs a
 // backend and that Info() reports the shaped "remote:path" location.
@@ -44,6 +61,107 @@ func TestNewRcloneStorerConstruction(t *testing.T) {
 	}
 }
 
+// TestRcloneStorerObjectAppearsOnlyWhenComplete checks that on a remote
+// where a file can be seen while it is still being written, such as local,
+// nothing is at the key until the upload has finished, so a killed upload
+// cannot leave a truncated object there.
+//
+//nolint:paralleltest // NewRcloneStorer installs the process-global rclone config
+func TestRcloneStorerObjectAppearsOnlyWhenComplete(t *testing.T) {
+	// Without this, rclone reads ahead of the write and holds a small
+	// upload in memory, so the progress callback would run before anything
+	// is written to the remote.
+	ctx, ci := fs.AddConfig(context.Background())
+	ci.BufferSize = 0
+	ci.StreamingUploadCutoff = 0
+
+	s, err := storage.NewRcloneStorer(ctx, ":local", t.TempDir())
+	if err != nil {
+		t.Fatalf("NewRcloneStorer: %v", err)
+	}
+
+	key := testBlobKey
+	data := bytes.Repeat([]byte("blob-bytes"), 1000)
+	seenEarly := false
+
+	err = s.PutWithProgress(ctx, key, bytes.NewReader(data), int64(len(data)),
+		func(int64) error {
+			_, statErr := s.Stat(ctx, key)
+			if statErr == nil {
+				seenEarly = true
+			}
+
+			return nil
+		})
+	if err != nil {
+		t.Fatalf("PutWithProgress: %v", err)
+	}
+
+	if seenEarly {
+		t.Error("object was at its key before the upload finished")
+	}
+
+	info, err := s.Stat(ctx, key)
+	if err != nil {
+		t.Fatalf("Stat after upload: %v", err)
+	}
+
+	if info.Size != int64(len(data)) {
+		t.Errorf("stored size = %d, want %d", info.Size, len(data))
+	}
+}
+
+// TestRcloneStorerListSkipsPartialFiles checks that a temporary file left
+// by a killed upload is never listed as a key.
+//
+//nolint:paralleltest // NewRcloneStorer installs the process-global rclone config
+func TestRcloneStorerListSkipsPartialFiles(t *testing.T) {
+	dir := t.TempDir()
+	ctx := context.Background()
+
+	s, err := storage.NewRcloneStorer(ctx, ":local", dir)
+	if err != nil {
+		t.Fatalf("NewRcloneStorer: %v", err)
+	}
+
+	realKey := testBlobKey
+
+	err = s.Put(ctx, realKey, strings.NewReader("blob-bytes"))
+	if err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	leftover := filepath.Join(dir, realKey+"-123456.partial")
+
+	err = os.WriteFile(leftover, []byte("half"), 0o600)
+	if err != nil {
+		t.Fatalf("writing leftover temp file: %v", err)
+	}
+
+	keys, err := s.List(ctx, "blobs/")
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+
+	if len(keys) != 1 || keys[0] != realKey {
+		t.Fatalf("List should return only the real key, got %v", keys)
+	}
+
+	var streamed []string
+
+	for obj := range s.ListStream(ctx, "blobs/") {
+		if obj.Err != nil {
+			t.Fatalf("ListStream: %v", obj.Err)
+		}
+
+		streamed = append(streamed, obj.Key)
+	}
+
+	if len(streamed) != 1 || streamed[0] != realKey {
+		t.Fatalf("ListStream should return only the real key, got %v", streamed)
+	}
+}
+
 // TestNewRcloneStorerUnknownRemote checks that a remote that is not in the
 // rclone config fails construction with the ErrRemoteNotFound sentinel,
 // rather than silently returning a backend pointed nowhere.

internal/vaultik/fault_injection_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -414,6 +415,69 @@ func TestBackupRetryAfterInterruptedUploadIsRestorable(t *testing.T) {
 	assertRestoredTree(t, fs, restoreDir, testFiles)
 }
 
+// Scenario 1c: a killed upload left a short object at a blob's key, as an
+// rclone remote that writes in place can. A later backup that packs the
+// same blob must upload it again rather than trust the short object. See
+// https://git.eeqj.de/sneak/vaultik/issues/266.
+func TestBackupReplacesShortBlobObject(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	fs := afero.NewOsFs()
+	tempDir := t.TempDir()
+	dataDir := filepath.Join(tempDir, "src")
+	restoreDir := filepath.Join(tempDir, "restored")
+	firstDBPath := filepath.Join(tempDir, "first.sqlite")
+	retryDBPath := filepath.Join(tempDir, "retry.sqlite")
+
+	ctx := context.Background()
+	cfg := faultTestConfig()
+	testFiles := writeFaultSourceTree(t, fs, dataDir)
+
+	store, err := storage.NewFileStorer(filepath.Join(tempDir, "remote"))
+	require.NoError(t, err)
+
+	firstDB, err := database.New(ctx, firstDBPath)
+	require.NoError(t, err)
+	fullFaultBackup(ctx, t, fs, store, cfg, database.NewRepositories(firstDB),
+		dataDir, firstDBPath, "first")
+	require.NoError(t, firstDB.Close())
+
+	blobKeys, err := store.List(ctx, "blobs/")
+	require.NoError(t, err)
+	require.NotEmpty(t, blobKeys)
+
+	shortKey := blobKeys[0]
+	require.NoError(t, store.Put(ctx, shortKey, strings.NewReader("cut off")))
+
+	// A backup with a new local index packs the same blobs again.
+	retryDB, err := database.New(ctx, retryDBPath)
+	require.NoError(t, err)
+
+	retryRepos := database.NewRepositories(retryDB)
+	id := fullFaultBackup(ctx, t, fs, store, cfg, retryRepos,
+		dataDir, retryDBPath, "retry")
+
+	blob, err := retryRepos.Blobs.GetByHash(ctx, path.Base(shortKey))
+	require.NoError(t, err)
+	require.NotNil(t, blob, "the retry must pack the blob that was cut short")
+	require.NoError(t, retryDB.Close())
+
+	info, err := store.Stat(ctx, shortKey)
+	require.NoError(t, err)
+	assert.Equal(t, blob.CompressedSize, info.Size,
+		"the short object must be replaced by the whole blob")
+
+	reader := newReaderVaultik(ctx, cfg, store, nil, fs)
+	require.NoError(t, reader.Restore(&vaultik.RestoreOptions{
+		SnapshotID: id,
+		TargetDir:  restoreDir,
+		Verify:     true,
+	}), "the retry's snapshot must be restorable")
+
+	assertRestoredTree(t, fs, restoreDir, testFiles)
+}
+
 // Scenario 2: the process dies during the metadata export, after the
 // database is uploaded but before the manifest. The destination is left
 // with blobs and a database but no manifest. verify and snapshot list